Fundamental transverse mode operation in solid state lasers

ABSTRACT

Fundamental transverse mode operation is achieved in a monolithic laser structure comprising an intracavity passive waveguide portion, which is capable of supporting only the fundamental transverse mode, arranged in tandem with the laser active medium, which typically would oscillate in a plurality of transverse modes. Particular embodiments are described for double heterostructure junction lasers.

3S50-96 1 3 SR uuuxu u Miller FUNDAMENTAL TRANSVERSE MODE OPERATION INsour) STATE LASERS Stewart Edward Miller, Locust, NJ.

Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Filed: Sept. 5, 1972 Appl. No.: 286,282

Inventor:

Assignee:

US. CL. 331/945 C, 331/945 H, 350/96 WG, 317/235 R Int. Cl. G02f 3/00Field of Search 33l/94.5 C, 94.5 H; 350/96 WG References Cited.

UNITED STATES PATENTS 7/1968 Snitzer et a1 331/945 OTHER PUBLICATIONSRipper et al.: Stripe Geometry Double Heterostruc- Feb. 5, 1974 tureLasers, Applied Physics Letters, Vol. 18, pp. 155-157, Feb. 15, 1971.

Morcatelli: Dielectric Rectangular Waveguide, Bell Systems TechnicalJournal, Vol. 48, pp. 207l-2102, Sept. 1969.

Primary Examiner-Edward S. Bauer Attorney, Agent, or Firm-M. J. Urbano[5 7] ABSTRACT Fundamental transverse mode operation is achieved in amonolithic laser structure comprising an intracavity passive waveguideportion, which is capable of supporting only the fundamental transversemode, arranged in tandem with the laser active medium, which typicallywould oscillate in a pluralityof transverse modes. Particularembodiments are described for double heterostructure junction lasers.

10 Claims, 2 Drawing Figures new 33l/94.5c

FUNDAMENTAL TRANSVERSE MODE OPERATION IN SOLID STATE LASERS of higherorder modes is assured by satisfying the inequality L2 2 2 w lltnBACKGROUND OF THE INVENTION 5 U This invention relates to solid statelasers and, more particularly, to arrangements for attaining fundamentalwhere w is the width of the higher order mode field in transverse modeoperation in such lasers. the active medium (i.e., approximately thestripe width It is well known in the laser art that for each longitu- Sin the junction laser case), A is the free space operatdinal (axial)mode at which a laser operates there are, ing wavelength of the laserradiation and n is the index in general, a plurality of associatedtransverse modes. of refraction of the passive portion surrounding theIn some applications, however, it is desirable that the waveguide. Thisconfiguration advantageously permits laser oscillate in only thefundamental or zero-order the use of wider stripes, thicker activeregions and transverse mode. For example, in an optical communihencehigher power densities and yet maintains fundacations system thefundamental mode facilitates optical l5 mental transverse modeoperation. coupling between various components of the system (e.g.,efficient matching into single-mode fibers for dis- BRIEF DESCRIPTION OFTHE DRAWING persion-free propagation). On the other hand, where Myinvention together with its various features and high power densitiesare desirable, such as in a laser advantages can be easily understoodfrom the following machining or micrographics system, the fundamentalmore detailed description taken in conjunction with the mode is mostsuitable because its energy distribution is accompanying drawing, inwhich: concentrated on the optic axis of the laser resonator. FIG. 1 isa schematic of an illustrative embodiment One type of laser particularlysuited to these applicaof my invention in which a waveguide ofrefractive tions is the double heterostructure (DH) junction laser indexn is surrounded by a region of uniformly lower which has been made tooperate on a continuous wave refractive index ri and basis at roomtemperature. For low power, c.w. appli- FIG. 2 is an end view of analternative embodiment cations the active region of the DH laser issufficiently of my invention in which a waveguide of refractive narrow(M2 to 1.0 um) that only the fundamental index n interfaces on itslateral sides with regions of transverse mode perpendicular to thejunction plane lower refractive index n;; and interfaces on its top andoscillates. Restriction of the oscillation to the fundabottom sides withregions of lower refractive index n mental mode parallel to the junctionplane is common] achieved b means ofa stri e contact eometr Howe ver,where higher power pu lsed outpu s are d DETAILED DESCRIPTION sired,catastrophic mirror damage considerations dic- Turning now to FIG. 1,there is shown a fundamental tate the use of thicker active regions(e.g., 2 to 4 pm). transverse mode junction laser 10 in accordance withUnfortunately, such thicker active regions permit an illustrativeembodiment of my invention. The laser higher order transverse modesperpendicular to the 10 comprises a double heterostructure (DH) portionof junction plane to oscillate. length Ll arranged in tandem andcontiguous with a passive portion of length L2. SUMMARY OF THE INVENTIONAs is now well known in the art, the DH portion typi- In accordance withan illustrative embodiment of my cally comprises an nGaAs substrate 12on which are invention, fundamental transverse mode operation is grownthe following epitaxial layers in the order reachieved in a monolithiclaser structure comprising an cited: intracavity passive w a eguidgpgrtpg disposed in tanan n-Al,Ga As layer 14, x 0; an n or p or dem andcontiguous with the active medium of a solid compensated Al, Ga As layer16, 0 s y x; and state laser. Whereas the active medium in general apA1,Ga, As layer 18, z y. The interfaces bemight generate radiation in aplurality of transverse tween layer 16 and layers 14 and 18 form a pairof modes, the dimensions (cross-section and length) and heterojunctions13 and 15, respectively, which define refractive index of the passiveportion are mutually an active region of thickness d, i.e., that regionwhere adapted to support the fundamental transverse mode low thresholdradiative recombination of holes and only. Reflections at the interfacebetween the active electrons occurs when the DH portion is forwardbimedium and the passive portion are reduced by either ased. The widthof the active region measured parallel of two illustrative techniques(1) by making the refracto the junction plane is limited by a stripegeometry tive index of the active medium and the passive portion metalcontact 22 of width S and length L1 which typias nearly equal aspossible or (2) by forming between cally extends between cleavedparallel surfaces 26 and the two a quarter wavelength impedance matching28 of the DH portion. Surface 26.may be made partially layer.transmissive to form an output face whereas surface 28 In anillustrative embodiment, the active medium is made contiguous with thepassive portion to be decomprises a stripe geometry p-n junctionsemiconducscribed hereinafter. The stripe geometry contact may tor bodyof the double heterostructure type and the passive portion comprises adielectric body of length L2 having therein an elongated rectangularwaveguide axially aligned with the active region of the DH body. Thedimensions and refractive index of the waveguide are mutually adapted tosupport only the fundamental be fabricated by well known oxide maskingtechniques or by a proton bombardment technique described in copendingapplication Ser. No. 204,222 (DAsaro Case 10-4-6-3) filed on Dec. 2,1971 and assigned to the assignee hereof. v

In order to facilitate making good electrical contact to the topside ofthe DH portion, a p-GaAs layer 20 is l transverse mode perpendicular andparallel to the junction plane. Adequate spreading, and henceattenuation,

interposed between layer 18 and stripe contact 22.

Contact to the bottom side is made through a metal contact 24 formeddirectly on substrate 12. Forward bias is illustratively provided bymeans of a battery 27 connected between contacts 22 and 24.

As mentioned above, a passive portion of length L2 is disposed in tandemwith the DH portion and contiguous with surface 28 thereof. The twoportions form a monolithic structure in which the end surface 30 of thepassive portion is disposed parallel to end surfaces 26 and 28. Surface30 is typically made highly reflecting to form an optical cavityresonator with surface 26. To this end, surface 30 may optionally beprovided with a well known reflecting coating (not shown).

With the cavity resonator thus defined, it is clear that both the DHportion and the passive portion are intracavity components. lmportantly,the passive portion is provided with an elongated core 32 having arefractive index n greater than the refractive index n of the remainderof the passive portion surrounding the core 32. Preferably, thecross-sectional shape of the core 32 is substantially congruent with thecross-sectional shape of the active region of the active medium. Thatis, in the case of the stripe geometry DI-I laser active medium shown inFIG. 1, the active region 34 is substantially a rectangularparallelepiped of crosssectional dimensions S X d and of length Ll.Accordingly, the core 32 is also a rectangular parallelepiped but ofcross-sectional dimensions W X D and of length L2. In general, becauseof the difference in the indicies of refraction and in the single-moderequirements of the passive portion, the dimensions W X D of the core 32need not be identical to the corresponding dimensions S X d of theactive region.

In operation, a higher order mode which might propagate in the activeregion 34 spreads upon entering the passive portion (as shown by lines35) and on reflection from the output face 30 returns greatly enlargedin cross section, thereby introducing appreciable loss in the resonantcavity for that mode. Adequate spreading of the higher order modes isassured by making the length L2 of the portion satisfy the inequalityback into the active region. The core will sustain the fundamentaltransverse mode if where and (2A/Dn 1.

At surface 28 there may be a reflection due to the difference betweenthe refractive index of the active region 34 and indices n and n, of thepassive portion. This would cause a reflection within the resonator. Theresulting coupled cavities may have the beneficial ef fect of spreadingthe longitudinal modes for the desired mode of propagation, provided thelengths L, and L are chosen properly. See, for example, an articleentitled Mode Selection in Lasers by P. W. Smith in Proc. IEEE, 60, 422(Apr. 1972). Alternatively, a layer may be interposed between the DH andpassive portions to provide an approximate impedance match between theactive region 34 and the core 32 using standard quarter-wave platematching techniques.

In practice, because of the asymmetry of the guided mode for theinjection laser structure, it might be desirable to reduce the opticalloss for radiation making the transition from the active to passiveportion at surface 28 by using the index configurations shown in FIG. 2for the passive portion. By having one index difference n -n in thevertical direction and another index difference nrn in the horizontaldirection, the single-mode properties and mode shape properties of thepassive portion can be more efficiently matched to the normal mode ofthe active region 34.

lllustratively, in the embodiment of FIG. 2, assume an injection laserwith a stripe width ofS 15pm operating at A 0.9 2m. The fundamentaltransverse mode of the active region 34 is expected to have a fieldvariation given by E cos (tr/231M833) where x is the transverse distancemeasured in micrometers from the center of the stripe in a directionparallel to the junction plane. Using glass with a refractive index of1.5 for the passive portion, this field shape can be matched quite wellwith an index difference (n n 1.5Xl0 and a core width of l3p.m. Thelength L2 28 001 calculates to be about 300p.m., A somewhat shorterpassive portion may prove adequate if the surface 30 is made absorbingoutside the region where the fundamental mode exists; this reduces thereflection for this higher order mode.

Alternatively, the passive portion may advantageously be made of GaAlAswith the core fabricated therein by well known photolithographictechniques, thereby avoiding any substantial refractive index differenceat surface 28. Thus, in FIG. 1, the core would comprise Al Ga, ,,As andthe surrounding passive portion would comprise Al Ga As with 0 s p q sothat n, n Of course, the aluminum content of the core i.e., the fractionp) is chosen to reduce reflections at surface 28.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of my invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and'scope of the invention. In particular, although specificallydescribed with reference to an injection laser, my invention isapplicable more generally to any multimode active region joined to asingle-mode passive region. Moreover, where desirable to reduce heatdissipation problems, the active medium may be provided with a suitableheat sink by means well known in the art.

There is being filed concurrently herewith a related application Ser.No. 286,283 in the name of B. W. Hakki (Case 9) entitled FundamentalMode, High Power Operation in Double Heterostructure Junction LasersUtilizing a Remote Monolithic Mirror and assigned to the assigneehereof.

What is claimed is:

1. In a solid state laser, a monolithic body comprising: an activemedium capable of producing stimulated coherent radiation of free spacewavelength A in a plurality of transverse modes when suitably pumped,said active medium having a first reflective face forming one mirror ofan optical resonator for sustaining said radiation and having a secondface, opposite said first face, through which said radiation istransmitted; a passive portion disposed contiguous with said second faceand in the path of said radiation, said passive portion having a thirdface, opposite said first face, forming another mirror of saidresonator, at least one of said first and third faces being partiallytransmissive to permit the egress of said radiation from said resonator,said passive portion including an elongated dielectric region thereinhaving a refractive index higher than the refractive index n of theremainder of said passive portion surrounding said dielectric region,said dielectric region having a cross-section perpendicular to the opticaxis of said resonator which is substantially congruent with the shapeof the cross-section of the active region of said active medium, thedimensions of the crosssection and refractive index of said dielectricregion being adapted to support only fundamental transverse modes, andthe length L2 of said dielectric region satisfying approximately thecondition: L2 2 2w /An where w is the width of the higher order modefield in said active medium.

2. The body of claim 1 wherein said elongated dielectric region is arectangular parallelepiped of width D axially aligned with said activeregion along the optic axis of said resonator and said remainder of saidpassive portion has a substantially uniform spatial distribution ofrefractive index n n said dielectric region satisfying the conditionthat A z (2MB)2 where A l n /n and I 2mm 1.

3. The body of claim 1 wherein said elongated dielectric region is arectangular parallelepiped of width D axially aligned with said activeregion along the optic axis of said resonator, and said remainder ofsaid passive portion is defined by four zones, upper and lower spacedzones of refractive index In n which are contiguous with a pair ofoppositely facing side surfaces of said dielectric region and a pair oflateral spaced zones of refractive index n; n which are contiguous withthe other pair of oppositely facing side surfaces of said 5. The body ofclaim 1 including an impedance matching layer formed on said second facebetween and contiguous with said active medium and said passive portion.

6. The body of claim 1 wherein said active medium comprises a doubleheterostructure including a pair of heterojunctions definingtherebetween said active region of thickness d and a stripe geometryelectrical contact of width S, said active region therebybeingsubstantially a rectangular parallelepiped of crosssectionaldimensions: width S and height d.

7. The body of claim 6 wherein said active medium and said passiveportion comprise AlGaAs.

8. The body of claim 6 wherein said active medium comprises AlGaAs, saidpassive portion comprises glass and including an impedance matchinglayer between and contiguous with said active medium and said passiveportion.

9. The body of claim 1 including means for pumping said active medium togenerate said radiation.

10. In a solid state laser adapted for fundamental transverse modeoperation, a monolithic body comprising:

a multilayered active medium including an epitaxial layer of nAl,Ga,,As, at least one epitaxial layer of Al Ga ,,As, O s y x, therebyforming a first heterojunction at the interface therebetween, and anepitaxial layer of pAl Ga As, z y, thereby forming a secondheterojunction at the interface between said Al Ga, ,As layer and saidAl Ga ,,As layer,

said medium including a planar p-n junction between said heterojunctionsand having a pair of spaced parallel cleavage surfaces one of whichforms one mirror of an optical resonator,

a stripe geometry electrical contact of length L2 and of width S formedon a major surface of said active medium and extending normal to saidcleavage surfaces, thereby defining an active region in the shape of arectangular parallelepiped of thickness d equal to the total thicknessof said at least one Al Ga ,,As layer, of width S, and of length L1,

a passive portion disposed contiguous with said second surface andhaving a third cleavage surface, parallel to said one mirror, forminganother mirror of said resonator, at least one of said first and thirdsurfaces being partially transmissive to radiation generated in saidactive region,

said passive portion including a waveguide region in the shape of arectangular parallelepiped axially aligned with said active region alongthe optic axis of said resonator, said waveguide region having lengthL2, height D, width W and refractive index n and the remainder of saidpassive portion having a refractive index n n said waveguide regionbeing designed so that where A is the free space wavelength of saidradiation,

and

uation of higher order modes,

(Z /Dm) 1,

said waveguide region compnsmg AI Ga, ,,As and thereby to support onlyfundamental transverse modes both parallel and perpendicular to theplane of said p-n Sald remainder p g q M-Q s P junction and furtherbeing designed so that the parameters p and q being adapted to reducere- L2 2 28 001 flections at said second surface.

thereby to assure adequate spreading and hence atten-

1. In a solid state laser, a monolithic body comprising: an activemedium capable of producing stimulated coherent radiation of free spacewavelength lambda in a plurality of transverse modes when suitablypumped, said active medium having a first reflective face forming onemirror of an optical resonator for sustaining said radiation and havinga second face, opposite said first face, through which said radiation istransmitted; a passive portion disposed contiguous with said second faceand in the path of said radiation, said passive portion having a thirdface, opposite said first face, forming another mirror of saidresonator, at least one of said first and third faces being partiallytransmissive to permit the egress of said radiation from said resonator,said passive portion including an elongated dielectric region thereinhaving a refractive index higher than the refractive index n2 of theremainder of said passive portion surrounding said dielectric region,said dielectric region having a cross-section perpendicular to the opticaxis of said resonator which is substantially congruent with the shapeof the crosssection of the active region of said active medium, thedimensions of the cross-section and refractive index of said dielectricregion being adapted to support only fundamental transverse modes, andthe length L2 of said dielectric region satisfying approximately thecondition: L2 > OR = 2w2/ lambda n2 where w is the width of the higherorder mode field in said active medium.
 2. The body of claim 1 whereinsaid elongated dielectric region is a rectangular parallelepiped ofwidth D axially aligned with said active region along the optic axis ofsaid resonator and said remainder of said passive portion has asubstantially uniform spatial distribution of refractive index n1 > n2,said dielectric region satisfying the condition that Delta about 3/4 (2lambda /D)2 where Delta 1 - n2/n1 and (2 lambda /Dn1) <<
 1. 3. The bodyof claim 1 wherein said elongated dielectric region is a rectangularparallelepiped of width D axially aligned with said active region alongthe optic axis of said resonator, and said remainder of said passiveportion is defined by four zones, upper and lower spaced zones ofrefractive index n2 < n1 which are contiguous with a pair of oppositelyfacing side surfaces of said dielectric region and a pair of lateralspaced zones of refractive index n3 < n1 which are contiguous with theother pair of oppositely facing side surfaces of said dielectric region.4. The body of claim 1 wherein the refractive indices of said dielectricregion and of said active region are substantially equal.
 5. The body ofclaim 1 including an impedance matching layer formed on said second facebetween and contiguous with said active medium and said passive portion.6. The body of claim 1 wherein said active medium comprises a doubleheterostructure including a pair of heterojunctions definingtherebetween said active region of thickness d and a stripe geometryelectrical contact of width S, said active region thereby beingsubstantially a rectangular parallelepiped of cross-sectionaldimensions: width S and height d.
 7. The body of claim 6 wherein saidactive medium and said passive portion comprise AlGaAs.
 8. The body ofclaim 6 wherein said active medium comprises AlGaAs, said passiveportion comprises glass and including an impedance matching layerbetween and contiguous with said active medium and said passive portion.9. The body of claim 1 including means for pumping said active medium togenerate said radiation.
 10. In a solid state laser adapted forfundamental transverse mode operation, a monolithic body comprising: amultilayered active medium including an epitaxial layer of n-AlxGa1 xAs,at least one epitaxial layer of AlyGa1 yAs, 0 < or = y < x, therebyforming a first heterojunction at the interface therebetween, and anepitaxial layer of p-AlzGa1 zAs, z > y, thereby forming a secondheterojunction at the interface between said AlzGa1 zAs layer and saidAlyGa1 yAs layer, said medium including a planar p-n junction betweensaid heterojunctions and having a pair of spaced parallel cleavagesurfaces one of which forms one mirror of an optical resonator, a stripegeometry electrical contact of length L2 and of width S formed on amajor surface of said active medium and extending normal to saidcleavage surfaces, thereby defining an active region in the shape of arectangular parallelepiped of thickness d equal to the total thicknessof said at least one AlyGa1 yAs layer, of width S, and of length L1, aPassive portion disposed contiguous with said second surface and havinga third cleavage surface, parallel to said one mirror, forming anothermirror of said resonator, at least one of said first and third surfacesbeing partially transmissive to radiation generated in said activeregion, said passive portion including a waveguide region in the shapeof a rectangular parallelepiped axially aligned with said active regionalong the optic axis of said resonator, said waveguide region havinglength L2, height D, width W and refractive index n1, and the remainderof said passive portion having a refractive index n2 < n1, saidwaveguide region being designed so that Delta about 3/4 (2 lambda /D)2where lambda is the free space wavelength of said radiation, Delta 1 -n2/n1) and (2 lambda /Dn1) << 1, thereby to support only fundamentaltransverse modes both parallel and perpendicular to the plane of saidp-n junction and further being designed so that L2 > or = 2S2/ lambda n2thereby to assure adequate spreading and hence attenuation of higherorder modes, said waveguide region comprising AlpGa1 pAs and saidremainder comprising AlqGa1 qAs, 0 < or = p < q, the parameters p and qbeing adapted to reduce reflections at said second surface.